Many printing processes utilize substrates, platens or forms as printing surfaces to transfer an image to a printable medium. One such process is called intaglio printing. Intaglio printing involves application of printed indicia or images below the surface of a platen or substrate that is utilized as a printing surface. Traditionally, intaglio substrates have been prepared by mechanically engraving or chemically etching a recessed pattern into the printing surface of the substrate, which defines an image. The pattern may comprise an array of dots in the printing surface of the substrate. The recessed pattern, such as the array of dots, define tiny recesses within which ink is held and transferred to the printable medium, such as a sheet or surface. This intaglio process is typically used in die stamping or in engraved processes, sometimes referred to as copper plate printing. It is also used in connection with pad printing, which is typically used to decorate plastic surfaces, as well as in the gravure printing process.
Mechanical engraving and chemical etching techniques are time-consuming processes. Mechanical techniques are typically slow due to limitations of engraving equipment. A mechanical stylus must be used to engrave the image into the substrate, which requires a certain amount of time to penetrate and cut the substrate material. Furthermore, accuracy of the engraving becomes an issue when the stylus becomes worn and dull. On the other hand, chemical etching is time consuming due to the many steps involved. Chemical etching is a multi-step process that first involves producing the image onto a film negative, such as with an imagesetter. Once the film is produced, it becomes a mask that can be laid on top of a copper or steel substrate having a thin film coating of sensitizing material. The substrate and mask combination is exposed to light for subsequent chemical development, which transfers the mask to the copper plate. After development, the substrate is ready for acid etching to complete the process. Accuracy is also an issue with chemical etching, due to the limited controllability of the chemical-etching process.
Another technique involves direct laser etching, which is a single-step process that requires much less time than mechanical engraving and chemical etching techniques. In this technique, a laser is used to directly engrave the substrate material. However, because metals have a high reflectance, the laser/metal interaction is not conducive to producing plates having sharp engravings. With metals and a majority of plastics, direct laser engraving causes the material to melt, which creates the recessed areas, but also creates pooling of melted material around these recessed areas. This pooling of material acts as a ridge surrounding the recessed areas, which adversely affects the accuracy and usefulness of the printing surface of the substrate. Thus, accuracy remains an issue. Furthermore, although the direct laser technique is only a single-step process, the speed of the engraving process still remains an issue at higher resolution levels, which require the laser to engrave a higher number of tightly focused dots to achieve such resolutions. With presently known systems, the engraving process time is increased when the resolution level is increased.
The system and method of the present invention addresses these and other problems associated with direct laser engraving of substrates.